Although the sound reproduction capabilities of mobile devices have improved substantially in the past years, partly due to the use of smart amplifiers, there is still a performance gap compared to external loudspeaker systems. Such portable loudspeakers are widely available in the market and can play audio content from mobile devices through a wireless (typically a Bluetooth™ link) or through a USB cable or a 3.5 mm jack audio cable. Wireless loudspeakers are active speakers that are externally powered by their own power sources. USB based speakers may be USB-powered or can have their own external battery pack or other power source. Portable loudspeakers using a 3.5 mm jack connection are generally battery-powered and can usually be charged using a USB adaptor cable. These wired battery-powered loudspeakers combine the drawbacks of a wired connection and of a battery-powered device. Adding a battery to the external speaker increases cost, size, weight, form factor and extra efforts for charging the batteries. Moreover, once batteries become non-functional, the whole device typically becomes unusable.
There also exist passive (not battery-powered) loudspeakers that can be directly connected to the 3.5 mm headphone jack socket. These do not suffer from the drawbacks of battery-powered loudspeakers, but the limited electrical output (typically max. 100 mW) of the headphone socket does not provide an acceptable sound reproduction level.
In current mobile phone architectures, the headphone amplifier is either a separate integrated circuit (IC) or integrated in an audio “CODEC” IC, which interconnects all audio components (hands-free loudspeakers, earpiece, headphones and microphones) with the main processor through digital and analogue audio interfaces.